Finely divided refined kaolin clay is widely used as a pigment to provide a glossy, white opaque surface finish on printing paper. The processing of refined kaolin pigments necessitates particle size fractionation and purifying crude clay while the clay is in the form of a fluid deflocculated aqueous slurry, bleaching the clay while in a flocculated state, filtering the flocculated clay to remove liquid water and then dispersing the flocculated clay in water to form a high solids slurry that is marketed as such or is dried, usually in a spray dryer, to provide a dry refined pigment capable of being mixed with water to form a dispersed fluid suspension. The latter form of clay is frequently referred to as a "predispersed" grade of clay even though the clay is dry and is not present in dispersed state until it is mixed with water.
Conventional kaolin clay pigments are applied onto paper sheets as aqueous "coating colors," which comprise a clay pigment, a dispersing agent for the clay, a suitable adhesive such as a polymer latex, starch, or mixtures thereof and other minor additives. Present-day coatings are applied at high machine speeds where the use of high solids coating colors is desirable. The formulation of coating colors at high solids requires the initial provision of fluid high solids suspensions or conventional "slips". These suspensions are subsequently mixed with adhesive dispersions or suspensions to prepare the coating colors. High solids suspensions of conventional hydrous (uncalcined) clays generally contain in excess of 65% clay solids (65 parts by weight dry clay to 35 parts by weight water). Typically, solids are about 70%. A dispersing (deflocculating) agent, conventionally a sodium condensed phosphate salt, or sodium polyacrylate, is present in the high solids suspensions of conventional coating clays in order to impart fluidity, since the systems do not flow in the absence of the dispersing agent.
Such kaolin clay pigments must meet certain requirements with regard to rheological properties and to the properties of the coated sheet material. The viscosity of the high solids suspension of the clay coating pigment must be sufficiently low to permit mixing and pumping. After the adhesive is incorporated, the resulting coating color must also have suitable viscosity for handling and application to the paper sheet. In addition, it is highly desirable to obtain a coated calendered sheet which has good opacity, gloss, brightness and printability.
It is conventional practice in the art to improve the opacifying or hiding power of coating colors by blending the clay pigments with more costly pigments having greater opacifying power, such as TiO.sub.2. The industry has long sought a kaolin clay pigment which imparts improved opacifying power to coated paper without sacrificing gloss and printability and which can preferably be used in the absence of other more expensive pigments.
High bulking clay pigments offer the opportunity of maintaining or improving the opacity, gloss and printability of coated paper at lower coating weights, thereby reducing the pigment cost for coating colors. Bulking pigments are those which provide coatings having high opacification at a low coat weight. Generally, bulking is achieved by introducing voids in a pigment structure which contribute to increased light scatter. Controlled calcination of kaolin clays results in one type of bulking clay pigment. Calcined bulked kaolin clay such as the material supplied under the registered trademark ANSILEX have enjoyed widespread commercial success for more than a decade. For many years attempts have been made to bulk hydrous kaolin clays, thereby avoiding the expense involved in calcining clay and also avoiding the increase in pigment abrasivity that appears to be an inherent result of calcination. U.S. Pat. Nos. 4,075,030; 4,076,548 and 4,078,941 teach procedures for increasing the opacifying power of hydrous kaolin clays by "selectively flocculating" ultrafine clay particles with a low molecular weight polyamine flocculating agent (e.g., ethylene diamine or hexamethylene diamine) or with long carbon chain amines or certain quaternary ammonium salts (e.g., "ditallowdimethyl" ammonium chloride) in the presence of a mineral acid flocculating agent, e.g., sulfuric acid, and optionally with the added presence of citric acid or mica or both. The selective flocculation treatment allegedly incorporates voids in the clay to form a low density, high bulking pigment which when used as a coating color pigment improves the opacity of paper coated therewith. U.S. Pat. No. 4,640,716 teaches the use of certain zirconium compounds such as zirconium ammonium carbonate to bulk clay.
Efforts to introduce bulking pigments to the paper industry have been thwarted by the poor rheology of slurries of these pigments. Generally, paper makers seek to use clay coating pigments capable of forming high solids clay-water slurries which have a low shear viscosity below 1000 cp, preferably below 500 cp, when measured by the Brookfield viscometer at 20 r.p.m.. High shear viscosity for these slurries should be such that they are no more viscous than a slurry having a Hercules endpoint viscosity of 500 r.p.m., preferably 800 r.p.m., using the "A" bob at 16.times.10.sup.5 dyne-cm. Those skilled in the art are aware that when using the Hercules viscometer and measuring endpoints of 1100 r.p.m. or higher, endpoint viscosities are reported in units of dyne-cm at 1100 r.p.m.; apparent viscosity increases as the value for dyne-cm increases. It is conventional to use the abbreviated term 'dyne". Thus, a "2 dyne" clay slurry is less viscous than a "9 dyne clay" slurry. As used hereinafter the expressions 500 r.p.m. or higher, or 800 r.p.m. or higher, are intended to include lower viscosities such that endpoint measurements are at 1100 r.p.m. and the values are reported as dynes.
Another problem encountered in the successful commercialization of bulking pigments obtained from kaolin clay has been the difficulty of producing a bulked structure that is sufficiently durable to survive during various stages of production and end-use and is also capable of being dispersed to form high solids clay-water slurries having acceptable viscosity. When the general wet processing scheme described above is employed to make bulked structures by adding a bulking agent before filtration, the bulked structure must still be present in the filter cake containing the bulked assemblages when the filter cake is "made down" into a fluid slurry. The expressions "make down" and "made down" are conventional in the industry and refer to the preparation of dispersed pigment-water slurries. In some cases, it may be necessary to apply mechanical work to the filter cake to reduce the viscosity to usable values. The bulked structure must be sufficiently tenacious to survive the mechanical forces during such treatment. Bulking pigments must also be sufficiently stable under the influence of shear to maintain the bulked structure under the high shear rates, such as the high shear rates encountered in pumping high solids clay water slurries in centrifugal pumps. Moreover, a bulked structure must be capable of being retained when the deflocculated clay water slurry is formed into a coating color using standard makedown equipment. Also, the bulked structure must survive during the coating application and subsequent calendering. The fragility of the bulked structures obtained by prior art chemical treatments of hydrous clay has limited their commercial use. As mentioned, commercial bulking clays heretofore used by the paper industry have been those prepared by calcining fine particle size hydrous clays. In such case, calcination "sets" the bulked structure such that it is sufficiently durable to survive manufacturing, handling and use. Generally, a criterion for durability of a bulked structure is the retention of improved opacification after the above-described handling.
Copending patent application, Ser. No. 861,943, filed May 12, 1986, provides new relatively inexpensive kaolin pigments with a stable bulked structure but also capable of being mixed with water and dispersants to form clay-water slurries and coating colors having usable low and high shear viscosity. Although aqueous suspensions of such bulked clay contain added deflocculant to increase fluidity, these suspensions are not truly in a fully deflocculated or dispersed condition because the bulked, flocced structure is retained. Thus, these slurries can be termed "partially deflocculated" or "partially dispersed" slurries or suspensions. The new bulked pigments, obtained using high charge density cationic polyelectrolytes to bulk the clay, can be applied to paper at low cost weights. Coated printing paper possessing superior printability, especially by rotogravure and offset methods, has been obtained. However, slurries of the improved bulked pigments tend to increase in viscosity when exposed to high temperature, for example when prepared into starch coating colors by the known steam jet cooking procedure. Similarly, viscosity may increase slowly upon exposure to elevated storage temperature (e.g. 100.degree. F.) over a period of several weeks. For example, when a bulked pigment obtained by treating kaolin clay with cationic quaternary ammonium polymer (Calgon 261 LV) is dispersed in water with a polyacrylate or polyphosphate dispersant, as described in Ser. No. 861,943, and the "dispersed" aqueous slurry is heated, the clay-water slurry thickens at about 60.degree. C. This occurs when such dispersants are either added to the washed filter cake obtained during processing or if the dispersants are added to previously spray dried bulked clay. It has also been found that slurries prepared by adding dispersant to conventionally washed (cold water) filter cakes of the new bulked pigments should be spray dried without aging the slurries for more than one or two days because high and low shear viscosity of spray dried pigments are adversely affected. When a hot wash is used, slurries can be aged for longer times, e.g., two weeks or longer without detriment to the viscosity of the pigment. By using a hot wash, pigments can be shipped in slurry form without an intermediate drying step. Such washing is not always feasible.
It is desirable to be able to ship high solids slurries of pigments in tank cars. A high degree of fluidity is required. In many instances, slurry shipments must be sufficiently fluid to flow out of tank cars under the influence of gravity alone. Such phenomena as thickening, gel formation and sedimentation are undesirable because they impair or prevent gravity flow.
It has been observed that the viscosity increase of fluidized slurries of cationically bulked kaolin slurries is at least partially due to the formation of a thixotropic gel. It has now been found that storage also may result in the formation of a sediment in the fluidized slurry of bulked clay. Gel and sediment formation are especially likely to occur when storage takes place at elevated temperatures. If the slurry is stirred, the gel breaks down and the slurry may have a low viscosity and flow freely. Nevertheless, the possibility of using gravity alone to unload tank cars is not certain.
Copending applications U.S. Ser. No. 06/908,439 and 07/040,918 address the problem of maintaining low viscosity of bulked kaolin pigments on storage, elevating the thermal thickening temperature, providing lower viscosity in hard water and reducing the viscosity of slurries in the presence of starch such as to permit steam jet cooking.
U.S. Ser. No. 06/908,439 teaches the use of a nonionic surfactant with a polyacrylate dispersant to solve some of these problems. U.S. Ser. No. 07/040,918 teaches the use of a relatively high molecular weight sulfonates, preferably with a polyacrylate dispersant for such purpose. Lignin sulfonates and naphthalene sulfonate formaldehyde complexes are disclosed as suitable sulfonates. The sulfonates were found to be more effective than nonionics in maintaining low coating color viscosities.
Sulfonates are known to be useful as auxiliary dipersants when used with polyphosphates (condensed phosphates). It is also well known that polyphosphates tend to revert as a result of hydrolysis to form phosphates that are not effective dispersants. This hydrolysis is accelerated by increase of temperature. Sulfonates have been used with polyphosphates to remedy this problem. See U.S. Pat. Nos. 3,341,340, 3,594,203, 3,519,454 and 3,736,165, Sawyer Jr. et al, and also U.S. Pat. No. 2,709,661 (Dietz), U.S. Pat. No. 3,130,063, Millman et al) and J. R. Hern and J. H. Fritz, "Auxiliary Dispersants for Pigments and Pigmented Coatings," TAPPI, December 1966, Vol. 49, No. 12, 77-88A.
The discovery that the use of nonionic surfactants or sulfonates along with a polyacrylate dispersant to fluidize slurries of bulked pigments overcomes problems previously recognized as being an obstacle to the commercial supply of bulked kaolin pigments in slurry form. A new problem has now surfaced. This problem results from the increased fluidity of slurries at the desired 62% shipping solids when using sulfonates to improve the thermal properties of slurries of pigments bulked with cationic polyelectrolyte and fluidized with a polyacrylate dispersant. Thus, appreciable settling of solids takes place when using a sulfonate with a polyacrylate dispersant.
A conventional approach to overcome settling of pigment slurries is simply to increase the solids contents of the slurries. This approach was not successful with slurries of chemically bulked kaolin pigments because increases in bulked pigments solids resulted in undesirable decreases in opacification. Furthermore, the tendency of pigment slurries to thicken or gel upon storage at elevated temperatures increased at these higher solids contents. It is known that colloidal thickening agents can retard or prevent sedimentation of dispersed slurries of coarse particle size kaolin slurries. See, for example, U.S. Pat. No. 3,130,063 to N. Millman et al. Thickening agents disclosed in this patent are high molecular weight organic carbohydrates which swell or dissolve in water. Listed agents are swollen starches, soaps, cellulose derivatives, karaya gum, guar gum, distearyl dimetlhy ammonium chloride, sodium polyacrylate and sodium alginate.